1. Field of the Invention
The present invention relates to a method of constructing a geomechanical model of an underground zone for coupling with a reservoir model to simulate fluid flows in the zone, from a geological model of the zone discretized by a fine grid.
2. Description of the Prior Art
The state of the art to which reference is made hereafter is described in the following books and publications:    Chin L Y, Thomas L K. Fully Coupled Analysis of Improved Oil Recovery by Reservoir Compaction. SPE Annual Technical Conference and Exhibition held in Houston, Tex. 1999; 393-401;    Ewing R E. Aspects of Upscaling in Simulation of Flow in Porous Media. Advances in Water Resources 1997; 20(5-6): 349-358;    Gutierrez M, Makurat A. Coupled HTM Modelling of Cold Water Injection in Fractured Hydrocarbon Reservoirs. International Journal of Rock Mechanics and Mining Science, 1997; 34: 3-4;    Koutsabeloulis N C, Heffer K J, Wong S. Numerical Geomechanics in Reservoir Engineering. Computer methods and Advances in Geomechanics, Siriwardane and Zaman eds., Balkema, Rotterdam, 1994;    Longuemare P, Mainguy M, Lemonnier P, Onaisi A, Gerard Ch, Koutsabeloulis N. Geomechanics in Reservoir Simulation: Overview of Coupling Methods and Field Case Study. Oil and Gas Science and Technology 2002: 57(5): 471-483;    Mainguy M, Longuemare P. Coupling Fluid Flow and Rock Mechanics: Formulations of the Partial Coupling between Reservoirs and Geomechanical Simulators. Oil and Gas Science and Technology 2002: 57(4): 355-367;    Renard P, de Marsily G. Calculating Equivalent Permeability: a Review. Advances in Water Resources 1997; 20(5-6): 253-278;    Reuss A. Calculation of the Flow Limits of Mixed Crystals on the Basis of the Plasticity of Mono-Crystals, Z. Angew. Math. Mech. 1929; 9: 49-58;    Salamon M D G. Elastic Moduli of a Stratified Rock Mass. J. Rock Mech. Min. Sci., 1968; 5: 519-527;    Settari A, Mourits F M. Coupling of Geomechanics and Reservoir Simulation Models. Computer Methods and Advances in Geomechanics 1994; 2151-2158;    Settari A, Walters D A. Advances in Coupled Geomechanical and Reservoir Modeling with Applications to Reservoir Compaction. SPE Reservoir Simulation Symposium, Houston, Tex. 1999;    Stone T, Garfield B, Papanastasiou P. Fully Coupled Geomechanics in a Commercial Reservoir Simulator. SPE European Petroleum Conference, Paris, France, 2000; 45-52;    Voigt W. Über die Beziehung zwischen den beiden Elastizitätskonstanten isotroper Körper. Ann. Phys. 1889, 38: 573-587;    Wen X H, Gomez-Hernandez J J. Upscaling Hydraulic Conductivities in Heterogeneous Media: An Overview. Journal of Hydrology 1996; 183.
Reservoir simulations allow oil companies to precisely estimate the reserves of an oil reservoir and to optimize recovery of these reserves by studying different production schemes. Prior to reservoir simulation, it is necessary to characterize the reservoir in terms of geometry and of petrophysical properties. This characterization provides a detailed description of the reservoir in the form of a geological model. The detailed description of the geological model is used to construct a model of multiphase flow in porous media through a scaling process. The various stages allowing going from reservoir characterization to reservoir simulation can be presented as follows:
characterization of the reservoir to construct a fine geological model (small cells);
scaling the fine geological model to construct a coarser reservoir model (larger cells); and
reservoir simulation from the reservoir model.
Characterization of the reservoir first requires its structural representation from surfaces (horizons and fault networks) describing the boundaries of structural blocks. This construction of the detailed geological model is carried out from the data provided by the wells and seismic data. The surfaces of the structural model are used as the basis for construction of a stratigraphic grid of finely gridded blocks as a function of geological criteria. The stratigraphic grids are thereafter provided with lithofacies and petrophysical properties (porosity and permeability). Assignment of the lithofacies and petrophysical properties is carried out from known well values and using geostatistical methods constrained by hypotheses on the geology of the sequences.
The geological model thus constructed provides a detailed description of the reservoir which cannot be used directly to model fluid flows in the reservoir. The reservoir grid used for modelling the flows is constructed from the geological model as a function of flow criteria. The scale difference between the reservoir and stratigraphic grids requires using scale change methods in order to define the petrophysical properties used for modelling the flows from information given at a smaller scale. Once this scaling stage is performed, reservoir simulation can be carried out.
Methods of this type are described for example in the aforementioned following publications: Wen and Gomez-Hernandez 1996, Ewing 1997, Renard and de Marsily 1997, and in patent application PCT WO-0/079,423.
The reservoir simulation carried out from the reservoir grid obtained after scaling models the multiphase flows of fluids in the reservoir. Some reservoir simulators furthermore have options allowing accounting for additional phenomena such as: thermal effects, constituents diffusion, double-porosity media processing. Besides, studying several little consolidated reservoir cases has shown the importance of the mechanical effects associated with the production of oil reservoirs. In fact, the reservoir model is more and more often coupled with a geomechanical model modelling the evolution of the stresses and deformations in the reservoir during production. The results of the geomechanical model are then used to modify the petrophysical properties of the reservoir model during reservoir simulation (Koutsabeloulis et al. 1994, Settari and Mourits 1994, Guttierez and Makurat 1997, Longuemare et al. 2002).
Coupling between the reservoir simulator and the geomechanical simulator can be achieved from two approaches described in the aforementioned references Settari and Mourits 1994, Settari and Walters 1999.
In a first approach, the flow and geomechanics problems are solved in the same simulator by internal coupling. This approach is for example adopted by Stone in the aforementioned publication Stone et al. 2000 and in U.S. Published patent application 2003/0,078,733. Taking account of the geomechanical behaviour appears therein as an option of the reservoir model.
In another approach, the flow and geomechanics problems are solved by external coupling of two simulators. Coupling is performed by data exchange between the reservoir and mechanical simulators as described, for example, in the aforementioned publications as follows: Settari and Mourits 1994, Chin and Thomas 1999, Mainguy and Longuemare 2002.
Whatever the approach used, modelling of the geomechanical a behavior of the reservoir generally comprises the following modelling stages:
a) construction of a geometry of the geological structure whose mechanical behavior is to be modelled;
b) gridding of the defined geometry;
c) assignment of the mechanical properties for each cell;
d) definition of initial boundary conditions and of a load record;
e) solution of the mechanical or thermo-poro-mechanical problem when the mechanical problem is coupled with thermal problems and/or problems of fluid flow in porous media; and
f) post-processing and analysis of the results.
The geometry used for the geomechanical model is generally the same at the reservoir level as that of the reservoir model. It is furthermore often vertically and laterally extended so as to account for the rocky formations surrounding the reservoir. The size of the cells used to discretize the geometry defined above is similar to (in the reservoir) or larger than (for the surrounding formations) that of the cells used for reservoir simulation.
The cells of the geomechanical model are in most cases large (of the order of one hundred meters in the horizontal directions and of ten meters in the vertical direction) whereas the rock heterogeneity is at the origin of a variability of the mechanical properties within the geomechanical cell. Now, the computing techniques used require definition of homogeneous mechanical properties for each geomechanical cell, whatever its intrinsic heterogeneity degree.
The procedure commonly adopted by engineers who perform geomechanical simulations is to assign uniform mechanical properties per cell, without taking systematically account of the rock heterogeneity at a smaller scale. The drawback of this procedure is that it does not depend, or arbitrarily only, on the description of the rock at a smaller scale, that it rests on no scientific basis such as, for example, a homogenization method, and that it induces errors in the prediction of the reservoir behavior during its development.